Field of the Invention
This invention relates to multimode optical fibers (MMFs) and, more particularly, to the design and manufacture of such fibers optimized for coarse wavelength division multiplexing (CWDM) applications.
Discussion of the Related Art
As discussed by Fleming et al. in U.S. Pat. No. 7,421,174 (2008), which is incorporated herein by reference, early investigators of MMF designs recognized that a parabolic refractive index profile in the core substantially reduced the intermodal dispersion in the fiber. However, they assumed that this parabolic profile would be optimum and that it would be the same for all transmission wavelengths and fiber compositions. This approach did not take into account the variation in refractive index dispersion in different material compositions from which the fibers were constructed. Around 1975, Keck and Olshansky recognized that the variation in dispersive properties of core and cladding materials in MMFs did affect the optimum profile shape for any wavelength of operation. They described the now standard representation used to calculate the optimum refractive index profile shape in optical fiber in U.S. Pat. No. 3,904,268 issued on Sep. 9, 1975, which is incorporated herein by reference. In this representation the refractive index nc(r) of the core at any radius, r, less than the core radius, α, is given bync(r)=nc1[1−2Δ(r/a)α]1/2  (1)where Δ=(nc12−nc22)/2nc12, nc1 and nc2 are the refractive indices of the core at r=0 and r=a, respectively, and λ is the operating wavelength of the system incorporating the optical fiber as a transmission medium. Prior to recognition of the impact of refractive index dispersion, αopt, the optimum value of the profile shape parameter α, was expected to be equal to two for all fiber transmission wavelengths.
This approach to MMF design is fraught with several difficulties. First, it requires that αopt be independent of wavelength over the entire operating bandwidth of the fiber. Second, it imposes the shape of the index profile [equation (1)] on the design process a priori.
In addition, Ge-dopant is commonly used to form the near-parabolic index profile in MMFs. While the Ge-doped index profile in MMFs can be optimized (via αopt, as above) to achieve a high bandwidth, the high material dispersion of Ge-doped silica limits the spectral width of the high bandwidth region. It is known that both P- and F-doped silica have much smaller material dispersion relative to Ge-doped silica, and fibers made with P- and/or F-dopants have much wider spectral width than conventional Ge-doped fiber. However, it is difficult to introduce a high P-dopant concentration during preform processing because P-doped silica has a high vapor pressure, and a significant fraction of P-dopant is burned off during preform collapse. It is also difficult to maintain a circular preform core containing a high P-concentration because it has much lower viscosity than the surrounding silica, typically a silica substrate tube.
Furthermore, upon exposure to either hydrogen or radiation, fibers containing a high P-concentration have a significantly higher added attenuation, which increases monotonically with the P-dopant concentration. Therefore, it would be desirable to limit the P-concentration in the fiber core.
Thus, a combination of dopants such Ge, P, Al, B, F is required to satisfy both the material dispersion properties imposed by the required CWDM operation as well as to resolve the above manufacturing issues. Typically, MMFs have been analyzed and designed using the so-called “α-profile” where the refractive index profile shape is parabolic. Such a procedure may be too restrictive to achieve effective CWDM-optimized MMFs while at the same time addressing process/manufacturing issues.